1. Field of the Invention
The present invention relates to a radio frequency voltage controlled oscillator (RF VCO). More particularly, the present invention relates to an RF VCO that is capable of effectively reducing phase noise.
2. Description of the Related Art
Phase noise is rapid, short-term, random fluctuations in the phase of a wave, caused by time domain instabilities and is affected mostly by 1/f noise of a current source, such as a metal oxide semiconductor (MOS) device. The 1/f noise denotes self-noise of a device caused by the reunion of electrons generated between silicon (Si) and silicon oxide (SiO2) when an electric current flows laterally from a source to a drain through a channel under a gate.
Phase noise has recently emerged as a major problem in designing a CMOS voltage controlled oscillator (VCO). This problem results from the 1/f noise in the current source of the VCO being up-converted into an LC tank of the VCO, thereby aggravating the phase noise. To prevent the up-conversion of the 1/f noise, additional active and passive devices are used.
FIG. 1 is a circuit diagram of a conventional radio frequency (RF) VCO. Referring to FIG. 1, an LC-type low pass filter, including an inductor Ld and a capacitor Cd, is interposed between a differential oscillator (OSC), including two metal oxide semiconductor field effect transistors (MOSFETs), and a current source SRC, including one MOSFET, such that the LC-type low pass filter may filter noise from a drain of the current source SRC.
FIG. 2 is a circuit diagram of another conventional RF VCO. Referring to FIG. 2, in addition to a first low pass filter connected to a drain of a current source SRC, a second low pass filter is connected to a source of the current source SRC by an inductor Ls. The second low pass filter prevents noise from coming through the source of the current source SRC.
The current source SRC may be configured in a cascade form by using two MOSFETs to reduce its channel length modulation, thereby preventing the up-conversion of the 1/f noise into an LC tank of the VCO. However, the configuration of the cascade-type current source SRC requires additional active and passive devices, which increase a size of the VCO as well as a unit price of the VCO, which is especially significant in a case of mass-production.
When the current source SRC is applied to a wireless communications terminal, there is also a high probability that an increase in a number of passive devices may cause a mismatching of inphase/quadrature (I/Q) signals generated by an output of the VCO in a direct conversion structure.